The invention relates to a filter medium in the form of a stable body which is made of ceramic material and is employed for filtering molten metals.
Revealed in the U.S. Pat. No. 3,524,548 is a solid porous filter for filtering molten aluminum; that filter is made of a baked granular-like refractory material which is not attacked by molten aluminum, and features a glassy type of binding agent containing not more than 10% silica.
The granulate material mentioned is "fused alumina" or "tabular alumina", e.g. a crushed corundum produced from a melt. The use of this material results in a filter of relatively low permeability and porosity. The effectiveness and filtering capacity of the filter is limited by the inner structure, for which reason in practice bundles of filter pipes are employed to achieve the desired throughput flow rate.
At the start and during filtration relatively large pressure differences are required in order for the aluminum to pass through the filter element.
By making use of a quite different type of filter element attempts have been made to eliminate such disadvantages as limited filtering capacity and large pressure differences. The U.S. Pat. No. 3,893,917 describes a filter element which is manufactured by impregnating a polyurethane foam with a ceramic suspension, expressing the excess suspension, drying and baking. With this method an exact reproduction of the structure of the original foam is obtained in solid ceramic form. Filter elements of this kind exhibit a high filter capacity and high throughput rate which permits them to be used in the form of simple filter plates. These filter elements suffer from the disadvantage that they are relatively expensive to produce. Also, they are relatively poorly wet inside by the molten metal passing through them.
Proposed is the manufacture of filter media wherein mainly spherical, refractory material, for example hollow corundum spheres are bonded together by sintering or by means of a chemical bond which can withstand high temperatures to form a solid porous body; also proposed is the use of these bodies for filtering molten metals.
If such a filter is employed for filtering molten metal, an excessive resistance to flow through the filter at the start can lead to an undesired, large difference in level between the run-in and run-out sides of the filter.
If the permeability of the filter is increased, for example by increasing the porosity, then the resistance to initial flow decreases; in general, however, the pore diameter, which essentially determines the effectiveness of filtration, is also increased resulting in less effective filtration.
As a rule all these known filter elements exhibit a tendency for poor wetting in the interior. As a result the rate of throughput is markedly reduced and the filtering capacity of the filter medium is not fully exploited.
The object of the present invention is to overcome the above mentioned disadvantages and to make available a filter medium which, as a result of as complete as possible wetting in the interior, exhibits as low as possible initial and filtering resistance at as high as possible filtering efficiency, throughput rates and filtering capacity.